The present invention relates to a thin film EL (electroluminescence) element with high contrast and good image quality.
Conventional thin film EL elements which have structures shown in FIGS. 1A and 1B are known.
The conventional thin film EL element shown in FIG. 1A is shown in FIG. 1 of "Structure and Characteristics of High-Brightness, Long-Life Thin Film EL Panel", pp. 84-104, NIKKEI ELECTRONICS, Nov. 18, 1974. In FIG. 1A, a transparent electrode 2 made of In.sub.2 O.sub.3, SnO.sub.2 or the like and a first insulating layer 3 made of Y.sub.2 O.sub.3, TiO.sub.2 or the like are sequentially formed by sputtering or electron beam deposition on a glass substrate 1. ZnS:Mn is deposited by electron beam deposition on the first insulating layer 3 using sintered pellets to constitute a luminescence layer 4. The amount of Mn added to the ZnS material varies in accordance with application purposes and normally falls within the range between 0.1 wt % and 2.0 wt %. A second insulating layer 5 of the same material as the first insulating layer 3 is deposited on the luminescence layer 4. A back electrode 6 made of Al or the like is deposited on the second insulating layer 5. When an electric field is applied between the transparent electrode 2 and the back electrode 6, this thin film EL element emits yellowish orange light.
Although this thin film EL element having the structure mentioned above has sufficient luminescence characteristics and long life in practical applications, a reflection coefficient between the second insulating layer 5 and the back electrode 6 is large, and incident ambient light is, therefore, reflected by 50% or more. When this EL element is used under high ambient illumination conditions, contrast ratio is decreased, resulting in inconvenience. In order to eliminate the above drawback, another conventional EL element is proposed, as shown in FIG. 1B.
The conventional thin film EL element shown in FIG. 1B is shown in FIG. 1 of U.K. Patent Application GB No. 2039146A. In FIG. 1B, a high-resistance light-absorbing layer 7 made of CdTe or the like is inserted between the luminescence layer 4 and the second insulating layer 5 of FIG. 1A so as to improve the contrast of the EL element. However, the luminescence characteristics of this EL element greatly differ from those of the EL element of FIG. 1A. Although the threshold voltage of light emission is lowered, the brightness slowly increases against voltage increase, resulting in a decrease in brightness. Moreover, when a high electric field is applied to the EL element shown in FIG. 1B, a dielectric breakdown often occurs.